The principal method used in the extraction of gold is cyanidation. The basic principle of the cyanidation process is that cyanide solutions have a preferential dissolving action for the precious metals contained in an ore (Marsden J., House I., The Chemistry of Gold Extraction, Chapter 6, published by Ellis Horwood Limited, 1992, page 259-308). The mechanism of cyanidation can be demonstrated by reaction (1) (Tukel, C., Celik, H., Ipekoglu, U., Tanriverdi, M., and Mordogan, H., Leaching of Ovacik gold ore with cyanide and thiourea, Changing Scopes in Mineral Processing, 1996, page 567-572).
4Au+8CNxe2x88x92+O2+2H2O4Au(CN)2xe2x88x92+4OHxe2x88x92xe2x80x83xe2x80x83(1)
The gold dissolution rate is believed to be dependent on the concentration of NaCN and the alkalinity of the solution. For efficient leaching, the gold should occur as free, fine-size, clean particles in an ore that contains no cyanicides or impurities that might destroy cyanide or otherwise inhibit the dissolution reaction.
The cyanidation process for the extraction of gold from different gold ores has been employed for nearly a century. Cyanide is a powerful lixiviant for gold and silver extraction from ores. But at the same time cyanide forms complexes with other metals, such as mercury, zinc, copper, iron, nickel and lead, which partially account for the consumption of cyanide in gold extraction. Also free cyanide as well as cyanide complexes of heavy metals are discharged to the tailings pond. Because of the toxicity of cyanide it is necessary to detoxify cyanide from the process stream before discharge.
The toxicity of free cyanide and metal cyanide complexes can manifest itself in either an acute or chronic manner. The period of acute toxicity ranges from a few minutes to several days. Although cyanide is not an accumulative toxicant, the metals bound to cyanide can bioconcentrate or bioaccumulate, resulting in permanent physiological damage.
In the western United States, gold operations are generally permitted with zero discharge from the processes. The state of Nevada which has the largest gold extraction operations in the United States, requires that operations not degrade the quality of the groundwater of the state. United States, requires that operations not degrade the quality of the groundwater of the state. Nevada sets the surface water quality at concentrations equal to state and federal drinking water standards with cyanide concentrations not to exceed 0.2 milligrams weak and dissociable (WAD) cyanide per liter. (Smith A. and Mudder T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal Books Limited, London, 1991; Bucknam C. H., Cyanide Solution Detoxification Jar Tests, The Minerals Metals and Materials Society, 1997, page 191-204).
In addition to the mining industries, cyanide wastes are also produced by the electroplating, chemical, petrochemical, metallurgical processing and tin production industries, for example. The toxicity of cyanide in wastewaters is related to its form and concentration. The chemistry of cyanide is complex and many forms of cyanide exist in mining solutions (Smith A. and Mudder T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal Books Limited, London, 1991). The major categories of cyanide compounds which are important from a toxicity viewpoint include: free cyanide; iron cyanide; weak acid dissociable (WAD) cyanides; and cyanide related compounds.
Mineral wastes are highly complex because of the chemical interactions occurring in the metallurgical processes, ore geochemistry, leaching reagents, meteorological factors, and site hydrology. The pH of cyanide leached slurries and process solutions are generally in the range of 9-11 and may contain high concentrations (beyond dischargeable limits) of cyanide, heavy metals and species which can form anionic complexes at high pH values viz., As, Mo and Se. Thiocyanate, cyanate and ammonia may also be present at levels of concern (Ritcey, G. M., Tailings Management: Problems and Solution in The Mining Industry. Process Metallurgy 6. Energy, Mines and Resources Canada, CANMET. Elsevier, 1989 pp 970).
Acid solutions favor the presence of HCN and at pH values below 7 essentially all of the free cyanide is present in CNxe2x88x92 form. At a pH value of 9.36 (equal to the pK), the concentrations of HCN and CN ion are equal. At lower pH values, and at 20xc2x0 C., most cyanide exists as molecular HCN: 69.6 percent at pH 9; 95.8 percent at pH 8; and greater than 99 percent at pH 7. It is apparent that most of the xe2x80x9cfree cyanidexe2x80x9dxe2x80x94the sum of molecular hydrogen cyanide and the cyanide ion in natural waters would be in the form of HCN (Marsden J., House I., The Chemistry of Gold Extraction, Chapter 6, published by Ellis Horwood Limited, 1992, page 259-308).
Simple cyanides are represented by the formula A(CN)x, where A is an alkali (sodium, potassium, ammonium) or metal, and x, the valence of A, represents the number of cyano groups present in the molecule (Huiatt, J. L., Kerrigan, J. E., Olson, F. A. and Potter, G. L. (editorial committee) Cyanide from Mineral Processing, Proceedings of a Workshop, Salt Lake City, Utah, 1982). Soluble compounds, particularly the alkali cyanides, ionize to release cyanide ions. The solubility is influenced by pH and temperature.
The complex alkali-metallic cyanides can generally be represented by the formula Ay M(CN)x, where A is the alkali, y is the number of alkalies, M is the heavy metal (ferrous or ferric iron, cadmium, copper, nickel, silver, zinc, or others) and x is the number of CN groups. The value of x is equal to the valence of A taken y times, plus the valence of the heavy metal. The soluble complex cyanides release the radical or complex ion M(CN)x rather than the CN group. The complex ion may then undergo further dissociation releasing cyanide ion. Metal cyanide complex ions may be considered as the soluble products of the reaction between the corresponding insoluble simple cyanide and excess cyanide ion.
In general, the complex ion is more stable than the original compound and thus subsequent dissociation is relatively minor. There are 18 elements that form complex cyanide compounds, and there are more than 64 oxidation states of these metals capable of forming complex cyanides under certain conditions (Smith A. and Mudder T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal Books Limited, London, 1991). Two factors influencing the rate of dissociation at a given temperature are the pH of the medium and the concentration of the complex ion. In general, dissociation rates increase with either decreasing pH or decreasing total cyanide concentration. As expected, the rates of dissociation differ among the complex cyanides.
Not only is the rate of dissociation dependent upon pH and concentration, but the degree of dissociation also depends strongly on pH and concentration, i.e., the production of free cyanide from the dissociation of complex cyanides. At acid pH, the percentages of free cyanides are high and decrease as pH is increased to neutral and alkaline values.
Some of the heavy metals present in tailings solution as a result of mineral dissolution or flotation reagent addition, notably iron, copper, and zinc, can be precipitated from simple salts as their hydroxides and settle within the tailing mass. Soluble cyano complexes of these metals, however, are not readily precipitated as hydroxides and remain in solution. Their removal generally requires more sophisticated treatment than simple hydroxide precipitation.
Table 1 describes the relative stabilities of cyanide complexes (Scott, J. S., An overview of Cyanide Treatment Methods for Gold Mill Effluents, Cyanide and the Environment Proceedings of a Conference, Tucson Ariz. 1984; Smith A. and Mudder T., The Chemistry and Treatment of Cyanidation Wastes, Mining Journal Books Limited, London, 1991). Total cyanide includes all the species of cyanide present in the solution.
Smith and Mudder 1991 and (Smith, A. Attenuation and Migration of Cyanide in the Environment, 1988) describe the Eh-pH diagram for the CNxe2x80x94H2O system.
The thermodynamics of the cyanide-cyanate reaction indicates that cyanate should be the predominant species under natural conditions. However, it has been found difficult to oxidize cyanide to cyanate under natural ambient conditions. A strong oxidant such as ozone, hydrogen peroxide, or chlorine is reported to be required to derive this reaction. Bacterial enzymes or catalytic surfaces of titanium dioxide, zinc sulfide and carbon have been reported to promote this oxidation as well.
There are many ways used to destroy cyanides. Some are briefly described below. Natural degradation is a simple way of destroying cyanides. The mill effluent water or slurry is allowed to stand in the tailing pond for an extensively long period of time. A combination of processes viz., volatilization, hydrolysis, photodegradation, dissociation, chemical and bacterial oxidation, and precipitation occur and oxidize the cyanide to cyanate. The process is a low capital and low maintenance operation, free of any toxic by-product formation. If residence time restrictions and threat to wildlife exist, then this process may not be suitable despite being the most simple and cost effective. However, natural degradation can be effective as a pretreatment to reduce treatment chemical consumption, though it is generally not sufficient by itself (Simovic, L., Snodgrass, W. J., Murphy, K. L. and Schmidt, J. W., Development of a Model to Describe the Natural Degradation of Cyanide in Gold Mill Effluents. Cyanide and the Environment Proceedings of a Conference, Tucson Ariz. 1984; Castri, K. F., MeDevitt, D. A. and Castric, P. A. (1981). Influence of Aeration on Hydrogen Cyanide Biosynthesis, Current Microbiology, v.5, 1981. pp 223-226)
The process of chemical oxidation, also known as the oxidation process, is of four major types: Alkaline Chlorination; Ozonation; Hydrogen Peroxide or Degussa Process; SO2/Air (INCO) Oxidation.
The process which was initially developed to treat the cyanide containing waste waters of the metal plating and finishing industry came to be the most widely used cyanide destruction process (Huiatt, J. L., Kerrigan, J. E., Olson, F. A. and Potter, G. L. (editorial committee) Cyanide from Mineral Processing, Proceedings of a Workshop, Salt Lake City Utah, 1982). The destruction of cyanide by alkaline chlorination may be accomplished by means of chlorine gas, calcium hypochlorite, or sodium hypochlorite.
In the ozonation process, ozone is generated electrically, either from air or from oxygen. Use of oxygen yields twice the ozone concentration at half the power, and there may be some oxidation contribution from the oxygen itself. Hydrogen cyanide, cyanide ion, the complexes of zinc, cadmium and copper as well as thiocyanate are reportedly quickly and easily destroyed. (Huiatt, J. L., Kerrigan, J. E., Olson, F. A. and Potter, G. L. (editorial committee) Cyanide from Mineral Processing, Proceedings of a Workshop, Salt Lake City, Utah, 1982).
Cyanide destruction may also be achieved with hydrogen peroxide (Ahsan, M. Quamrul, PhD. 1990, Degussa Corp.). Copper has been reported to act as a catalyst in bringing about the oxidation of cyanide to cyanate in the presence of hydrogen peroxide.
The INCO process sparges the cyanide solution with SO2 in an air stream. In the INCO process, two to five percent SO2 is sparged into the solution containing at least 50 mg/L Cu2+. The copper can either be from the Cu present in the waste stream, or added as copper sulfate. The SO2 can be supplied as SO2 or as soluble sulfite or meta-bisulfite salt. Ferricyanide, if present, is reduced to ferrocyanide and precipitated as an insoluble metal ferrocyanide, M2Fe(CN)6 where M can be Cu, Ni, or Zn.
The acidification/volatilization/reneutralization treatment has been researched extensively (Huiatt, J. L., Kerrigan, J. E., Olson, F. A. and Potter, G. L. (editorial committee) Cyanide from Mineral Processing, Proceedings of a Workshop, Salt Lake City, Utah, 1982). Hydrogen cyanide is extremely volatile, with a vapor pressure of 100 kPa at 26xc2x0 C. This phenomenon is utilized in the Mills-Crowe process for cyanide regeneration. The solution is acidified (0.50-1.0 g/l H2SO4 excess) and dropped through a grid-packed tower counter-current to an air stream. The air, which picks up the hydrogen cyanide formed by the acid from the cyanide ion and zinc, copper and nickel cyanide complexes, is swept into an absorber tower where it contacts a weak lime slurry dispersed as a mist. The absorber tower solution is recycled so as to build up useable levels of cyanide concentration for return to cyanidation. The acid treatment does not liberate cyanide from ferrocyanide or thiocyanate. A significant amount of solid, consisting of gypsum (from the neutralization of lime by sulfuric acid) and possibly some calcium carbonate from the CO in the air, will form in the depleted cyanide solution. Depending on the composition of the barren feed and the final pH of the treated solution, cuprous cyanide and thiocyanate, and zinc, copper, nickel and iron may also be present in solution. In addition the solution is, of course, acid. It is therefore necessary to neutralize and filter the stripped, treated barren solution before discharging it as effluent.
Adsorption can be achieved by the following processes: Ion Exchange; Activated Carbon; Ion Flotation; and Precipitation Flotation.
The ion exchange process for cyanide recovery originated in 1956. Its development was prompted as much by concern for recovery of water in a relatively arid region as by loss of cyanide. The system consists of a lead column of the anion exchange resin to adsorb cyanide complexes, followed by a column of the same resin conditioned by precipitation of cuprous cyanide in the resin matrix, to remove free cyanide. The process apparently operates at about pH 11. The indications are that all the metal complex cyanides are in fact readily adsorbed by anion exchange resins, and since there is often little or no uncomplexed cyanide ions in gold mill effluents, this is often not a matter for concern. Where this is not the case, a small amount of any of the metals (zinc, copper or ferrous ion) could be added. High thiocyanate concentrations can interfere with cyanide adsorption.
The use of activated carbon for cyanide removal dates back some 14 years and apparently stems from attempts to employ it as a catalyst for the oxidation of cyanide. It was found that cyanide was first adsorbed, then catalytically oxidized. The presence of cupric ions results in the formation of copper cyanides, which enhances the adsorption capacity of the carbon. This permits faster flow rates and adds to the catalytic action. The copper may be impregnated on the carbon or fed with the cyanide solution. Continuous copper feeding causes hydrolysis of cyanate to yield ammonia and carbon dioxide (Huiatt, J. L., Kerrigan, J. E., Olson, F. A. and Potter, G. L. (editorial committee) Cyanide from Mineral Processing, Proceedings of a Workshop, Salt Lake City, Utah, 1982). Battelle Institute investigated the removal of cyanide using granular activated carbon (20xc3x9750 mesh) without oxidation. They also found it necessary to add copper (or nickel), this time to ensure removal. The presence of dissolved oxygen thus becomes unnecessary but the bed must then be regenerated.
Ion flotation has been known for about 20 years. It resembles conventional froth flotation in that it employs a collector and similar equipment, and the substance to be separated is carried out of the aqueous medium as a froth of air bubbles. It differs from froth flotation in that the substance to be separated is not usually present initially as a solid. The collectors are ionizable surface-active organic compounds, cationic for the flotation of anions, anionic for the flotation of cations. Since cyanide and its metal complexes are anions, cationic collectors are required. These are usually organic amines similar to those used for liquid-liquid extraction. Thus the mechanism of collection is similar to that of solvent extraction.
Precipitation flotation differs from ion flotation in that a colloidal precipitate is first formed and then floated. A laboratory investigation reported that ferrocyanide and nickelo-cyanide could be floated effectively. Cuprocyanide has been reportedly successfully removed from a zinc concentrate thickener overflow on an industrial scale (2200 m3/day) by a combination of ion precipitate and ultra-fine particle flotation, at the Kamioka mine in Japan.
Electrochemical methods for treatment of cyanide can be divided into three categories: Electroreduction; Electrooxidation; and Electrochlorination. With the electroreduction (cathodic) reaction, complex metal cyanide ions undergo reduction at the cathode to deposit or precipitate the metal, generating a corresponding amount of cyanide ion. Electroreduction permits recycling of the regenerated cyanide in the treated barren solution to cyanidation to the extent that the water balance will accommodate it. Electrooxidation (anodic) reactions have apparently not been established for treatment of cyanide. In electrochlorination, introduction of sodium chloride into the solution to be treated gives rise to active chlorine either at the electrode or in solution. These react with cyanides to form cyanates, and with thiocyanate to form cyanate and sulfate, as in conventional alkaline chlorination. Chloride ion is regenerated and is therefore again available for charge transfer. The electrolysis should be carried out at 40xc2x0-50xc2x0 C. to minimize formation of chlorate at the expense of hypochlorite generation. As with conventional alkaline chlorination, pH control is important. The ability of electrooxidation and electrochlorination to reduce total cyanide concentration in the effluents to the proposed levels has not yet been demonstrated unequivocally. As far as is known, none of these electrochemical options results in elimination of ferrocyanide.
The addition of excess ferrous ions to solutions of free cyanide converts most of the cyanide to ferrocyanide at an alkaline pH of 10 or above. This is one of the oldest cyanide disposal methods (Goodwin, Ernest, xe2x80x9cProcess for the Removal of Cyanide and Other Impurities from Solution,xe2x80x9d U.S. Pat. No. 4,840,735, Jun. 20, 1989; Isamu, Kato, xe2x80x9cRemoval of Cyanides From Wastewaters by Controlled Addition of Ferrous Salts.xe2x80x9d Japanese Patent, patent number: 04 83,590, Mar. 17, 1992; Shutt, Thomas C., xe2x80x9cMethod for Detoxifying Cyanide-containing Water,xe2x80x9d U.S. Pat. No. 5,264,192, Nov. 23, 1993). The process reactions are as shown below:
Complexation of Free Cyanides ore:
6CNxe2x88x92+3Fe2+xe2x86x92 greater than Fe2Fe(CN)6
Other Complexation of Metal Cyanides ore:
3Cu(CN)32xe2x88x92+3Fe2+xe2x86x92 greater than 3CuCN+Fe2Fe(CN)6
3Zn(CN)42xe2x88x92+3Fe2+xe2x86x92 greater than 3Zn(CN)2+Fe2Fe(CN)6
Ferrocyanide is a very stable complex. It settles down at the bottom of the tailings pond and it is temporarily non-toxic. Presence of thiocyanates drastically hampers the complexation process and the removal is not achieved to an acceptable level. Recently it has been reported that the ferrocyanides decompose under sunlight yielding HCN and the stability of ferrocyanide is questionable.
Bio-degradation has been successfully applied at Homestake Mining""s Lead, South Dakota using rotating biological contactors (Mudder, T. I. and Whitlock, J. L., 1983 xe2x80x9cBiological Treatment of Cyanidation Wastewaters, Paper in Proceedings of the 38th Industrial Wastes Conference, Purdue University, 1983, pp 279-287; Whitlock, J. L. and Mudder, T. I., 1986, The Homestske Wastewater Treatment Process: Biological Removal of Toxic Parameters From Cyanidation Wastewaters and Bioassy Effluent Evaluation, Chapter in Fundamental and Applied Biohydrometallurgy, ed. by R. W. Lawrence, et al., Elsevier, 1986, pp. 327-339). It is well known that certain microorganisms, such as fungi and bacteria, can metabolize cyanide. Bacilus megaterium converts KCN to asparagine, aspartic acid and carbon dioxide. On the other hand, Pseudomonas paucimobilis mudlock oxidizes free and complexed cyanide to carbonate and ammonia. The U.S. Bureau of Mines at Salt Lake City has cultured a different strain of bacteria Pseudomonas pseudoalcaligenes from a tailing pond water, which is reportedly capable of oxidizing cyanide better than Pseudomonas paucimobilis mudlock. Various biological processes such as trickling filters, activated sludge, fluidized bed reactors and rotating biological contactors (RBC""s) have been studied for their suitability for application to cyanide decomposition.
There is a continuing need for a process that renders a cyanide or cyanide compound substantially insoluble in cyanide-containing materials.
A method for rendering a cyanide-containing compound substantially insoluble in an aqueous solution or suspension of cyanide-containing materials or a solid cyanide-containing material comprising: mixing a reagent comprising a thiosulfate salt with said solution or suspension; and adding to said material a complexing agent selected from the group consisting of divalent copper salts, divalent iron salts, divalent cobalt salts, activated carbon, and mixtures of the foregoing is provided. Preferred complexing agents are: copper sulfate, ferrous sulfate, cobalt sulfate, activated carbon, and mixtures of the foregoing. The thiosulfate salt is preferably selected from the group consisting of ammonium thiosulfate, sodium thiosulfate, potassium thiosulfate and mixtures of the foregoing.
The method may further comprise separating solids containing substantially insoluble cyanide-containing material from liquids.
This invention provides methods for the detoxification of cyanide. By xe2x80x9cdetoxificationxe2x80x9d is meant oxidation thereof or rendering it substantially insoluble by forming strong complexes with other ions. These complexes are even stronger and less soluble than the xe2x80x9cstrong complexesxe2x80x9d currently known to the art, i.e. Fe(CN)64xe2x88x92, Co(CN)64xe2x88x92, and Au(CN)2xe2x88x92. The material to be treated by the cyanide detoxification process of this invention includes free cyanide, weak acid dissociate (WAD) cyanide, including readily soluble salts and relatively insoluble salts of cyanide, weak complexes, moderately strong complexes and strong complexes.
A preferred embodiment of the method comprises mixing a reagent comprising a thiosulfate such as ammonium thiosulfate, sodium thiosulfate, potassium thiosulfate, any copper thiosulfate and mixtures thereof, with an aqueous solution or suspension of cyanide-containing materials. At least stoichiometric amounts of thiosulfate based on the amount of cyanide in the solution or suspension is preferably used, preferably at least about two to one hundred (ppm) times the cyanide (ppm) present in the solution and in the slurry or sludge. Greater or lower amounts of thiosulfate may be used, as long as the desired amount of cyanide detoxification occurs. This may be achieved by changing other experimental parameters, as known in the art using the teachings described here.
To this mixture is added a complexing agent selected from the group consisting of divalent copper salts, divalent iron salts, divalent cobalt salts, activated carbon, and mixtures thereof. At least stoichiometric amounts of the salts based on the amount of cyanide in the solution or suspension is preferably used, preferably at least about one to about fifty (ppm) times more than the cyanide (ppm) present in the solution and in the slurry or sludge. The salts may be any readily available salts including as anions, sulfate, chloride, nitrate and metallic salts. Greater or lesser amounts of complexing agent may be used, as long as the desired amount of cyanide detoxification occurs.
Also provided is a composition comprising: between about 10 and 2000 ppm of a thiosulfate salt; and between about 1 and 1000 ppm of a complexing agent selected from the group consisting of divalent copper salts, divalent iron salts, divalent cobalt salts, activated carbon, and mixtures of the foregoing.
Activated carbon may be of any readily-available commercial particle size and grade, e.g. smaller than about 4 mesh, and preferably between about 4 and about 100 mesh. Sufficient activated carbon should be used to adsorb all the cyanide ions present. A weight ratio of activated carbon to cyanide in solution between about I and about 10 is preferred.
The pH of the reaction mixture will typically be the pH of solutions or suspensions produced as intermediate or product streams of gold recovery processes. The pH should preferably be maintained between about 5 and about 11 and all intermediate ranges therein, such as by the addition of acids or bases such as NaOH, CaO or Ca(OH)2 as and if required. The pH of the reaction mixture is most preferably maintained to between about 8 and 10 during the reactions of the methods of the invention, and all intermediate ranges therein, by means known in the art.
The Eh of the solution can be adjusted to between about 0.2 and 0.8V, and all intermediate ranges therein, using bleach/oxygen/H2O2 or other methods known in the art.
The reactions may be carried out for any time sufficient to cause the desired detoxification of cyanide. If a sequence of reagents is used, the reaction times with each reagent do not need to be the same. Preferably, the sample is allowed to contact the thiosulfate for between about 5 and 50 minutes, and the sample is allowed to contact the complexing agent for between about 2 and 20 minutes.
The reaction may be carried on at temperatures between about 10 and about 50xc2x0 C., preferably between about 20xc2x0 C. and about 25xc2x0 C.
The cyanide-containing materials to which this process may be applied may be cyanide solutions, sludges, or dry materials such as dried sludges. Sample preparation methods are known in the art, described herein, or readily determinable by one of ordinary skill in the art using methods known in the art or described herein. For example, when dried materials are treated, sufficient water must be added to form a medium in which the cyanide insolubilization reactions can take place. Preferably a volume ratio of water to dry material of at least about 1 is used.